The present invention relates generally to ceramics and more particularly, to spherical particles of boron nitride in the micron size range and to a method of preparing them.
Epoxy filler is widely used in the production of chip mountings for integrated circuits packages. The filler is a mixture of thermally conductive particles that are added to an epoxy resin to produce a particle/resin mixture. Injection molding is the preferred process for forming chip mountings from the particle/resin mixture, which places constraints on the physical form of the particles used as filler for an efficient injection molding. In particular, flat or flaky particles tend to stack during injection molding. This stacking increases the viscosity of the mixture during injection molding, which can lead to the production of air spaces in the epoxy/particle mixture that remain after the mixture cures. Air is a poor conductor of heat, and air spaces in the chip mounting attenuate the thermal transfer properties of the resulting chip mounting. Spherical particle filler is preferred over flat and flaky filler since spherical particles do not stack during injection molding.
The present market for spherical ceramic particles is dominated by spherical silica (SiO2) because silica spheres are available in a wide variety of sizes at low cost. However, the thermal conductivity, electrical conductivity, and other properties of silica are not optimal for chip mountings and other applications. Alumina (Al2O3) is preferred over silica since it has a higher thermal conductivity and a lower electrical conductivity. Importantly, alumina can also be produced in the form of micron-sized spherical particles (see, for example: H. Shim et al., xe2x80x9cRestructuring of Alumina particles Using a Plasma Torchxe2x80x9d, J. Mat. Res., volume 14, page 849 (1999); C-K Chen et al. J. Mat. Res., vol. 16, p. 1256, (2001); U.S. Pat. No. 5,989,648 to J. Phillips entitled xe2x80x9cPlasma Generation of Supported Metal Catalysts,xe2x80x9d issued on Nov. 23, 1999; and U.S. patent application Ser. No. 09/637,172, to Phillips et al., all incorporated by reference herein).
The current belief is that the heat transfer properties of a chip mounting depend significantly on the thermal conductivity of the filler particles used. Boron nitride (BN) has the highest thermal conductivity of any known non-electrically conductive ceramic. If spherical particles of BN of the appropriate size could be made at a reasonable cost, they would likely replace spherical silica or spherical silica and/or spherical alumina for at least some applications, such as for filler for integrated circuit packages. Thus, there is a need for spherical crystalline or at least partially crystalline particles of BN in the micron size range. However, no such method has yet been reported.
Therefore, an object of the present invention is to provide a method for generating spherical particles of boron nitride in the micron size range
Another object of the present invention is to provide spherical particles of boron nitride in the micron size range.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention includes a method for producing crystalline or partially crystalline particles of boron nitride, comprising the steps of generating an aerosol comprising precursor particles of boron nitride suspended in an aerosol gas; generating a plasma from a plasma gas, the plasma comprising nitrogen atoms, the plasma including a plasma hot zone having a temperature sufficiently high to melt boron nitride; directing the aerosol into the plasma hot zone and allowing the precursor particles of boron nitride to melt; and allowing the molten particles to exit the hot zone, whereby they cool and solidify to form crystalline or partially crystalline product particles of boron nitride.
The invention also includes a method for generating larger particles of boron nitride from smaller particles of boron nitride, comprising the steps of generating an aerosol comprising precursor particles of boron nitride suspended in an aerosol gas; generating a plasma from a plasma gas, the plasma comprising nitrogen atoms, the plasma including a plasma hot zone having a temperature sufficiently high to melt boron nitride; directing the aerosol into the plasma hot zone and allowing the precursor particles of boron nitride to melt, collide, and join to form larger particles; and allowing the molten particles to exit the hot zone, whereby they cool and solidify to form solid particles of boron nitride that are larger than the precursor particles.
The method also includes a method for melting boron nitride, comprising the steps of generating a plasma comprising nitrogen atoms, the plasma including a plasma hot zone having a temperature sufficiently high to melt hexagonal phase boron nitride; and exposing precursor particles of boron nitride to the plasma, whereby boron nitride melts.
The invention also includes crystalline or partially crystalline boron nitride particles made by the process comprising the steps of generating an aerosol comprising precursor particles of boron nitride suspended in an aerosol gas; generating a plasma from a plasma gas, the plasma comprising nitrogen atoms, the plasma including a plasma hot zone having a temperature sufficiently high to melt boron nitride; directing the directing the aerosol into the plasma hot zone and allowing the precursor particles of boron nitride to melt; and allowing the molten particles to exit the hot zone, whereby they cool and solidify to form crystalline or partially crystalline solid particles of boron nitride.
The invention also includes crystalline or partially crystalline boron nitride particles made by the method comprising the steps of generating an aerosol comprising precursor particles of boron nitride suspended in an aerosol gas; generating a plasma from a plasma gas, the plasma comprising nitrogen atoms, the plasma including a plasma hot zone having a temperature sufficiently high to melt boron nitride but not high enough to decompose the boron nitride; directing the aerosol into the plasma hot zone and allowing the precursor particles of boron nitride to melt, collide, and join to form larger particles; and allowing the molten particles to exit the hot zone, whereby they cool and solidify to form crystalline or partially crystalline solid particles of boron nitride that are larger than the precursor particles.
The invention also includes crystalline or partially crystalline spherical particles of boron nitride having a diameter of about 1-1000 microns.